The present invention pertains generally to a noise filter, more specifically to a filter particularly suited for suppressing noise and ripple components in power supply circuits and power circuits for analog and digital ICs, or the like.
Electronic circuits are normally designed to operate on a DC power supply voltage obtained from a battery, or AC power line in conjunction with a rectifier circuit. DC-DC converters (also known as switching type regulators) are commonly used for obtaining one or more DC voltages of any desired magnitude. Transient currents caused by switching or other electrical phenomena, including load fluctuations due to the switching operation of digital ICs or the like, introduce large amount of noise of various frequency components into power supply circuits. Such noise may cause malfunctions or reduce the S/N ratio of other nearby electronic equipment or appliances through lower supply line or electrical radiation if they are not suppressed.
Noise filters are used to protect or reduce such noise. Additionally, industry standards and governmental regulations have become increasingly strict because of the noise interference effects on a variety of high performance and sensitive electronic equipment now in use.
Almost all commercially available noise filters are of two types: a combination of discrete inductors and capacitors, or lumped LC (inductance-capacitance) device comprising a capacitor and bead cores mounted around its lead wires to increase inductance. Within the last five to ten years a distributed constant type noise filter has appeared in the literature. An example of such a filter is the subject of the present applicant's U.S. Pat. No. 4,563,658 that issued on Jan. 7, 1986, and is illustrated in FIGS. 7A-7C of the present specification. FIG. 7A shows the construction details of this type of noise filter, namely a belt-shaped current carrying conductor 3 and similar ground conductor 5 such as aluminum laminated with relatively wide dielectric sheets 6 and wound in a generally tubular form 10 as shown in FIG. 7B. Coupled near both ends of the current carrying conductor 3 are lead wires 1 and 2. Also coupled near one end of the ground conductor 5 is a ground lead wire 4. When spirally wound, an inductance is formed between both lead wires 1 and 2, the value of that inductance being proportional to the number of turns that the conductors are wound into and the length of the current carrying conductor 3. Also, a large capacitance occurs between the current carrying conductor 3 and the ground conductor 5 as a result of their proximity, i.e. being spaced apart from each other by a thin dielectric sheet 6. A schematic representation of the equivalent circuit of the resulting distributed constant type noise filter is shown in FIG. 7C.
Such distributed constant type noise filters are relatively simple and yet have relatively good attenuation characteristics over a wide frequency range. To date, no one has attempted to commercially make such noise filters. The primary reasons include difficulty in making electrical connections to the at least three lead wires because of the relatively small size of the filter. Low production yield is another problem and thus, its previous unsuitability to quantity production because of large differences in attenuation characteristics and lead wire misalignment.
It is desirable to have a noise filter that is inexpensive and easy to produce that consistently has good attenuation characteristics and lead wire alignment. The present invention provides such a noise filter and a method of producing same.